Conventional differential feedback amplifiers suffer one or more problems, including relatively high power consumption, relatively narrow dynamic range, relatively low linearity and a lack of matched input and output impedances requiring additional impedance matching circuitry. Attempts to compensate for these problems often exacerbate other problems.
FIG. 1 illustrates an exemplary conventional differential feedback amplifier with compensation disclosed in U.S. Pat. No. 4,835,488, entitled, Wideband Linearized Emitter Feedback Amplifier.” Conventional amplifier 100 comprises main amplifier 110 and compensation amplifier 120. Main amplifier 110 comprises bipolar transistors QMP1, QMP2, emitter feedback resistor RME, load resistors RL1, RL2 and active tail current sources IM1, IM2. Compensation amplifier comprises bipolar transistors QCP1, QCP2, emitter feedback resistor RCE, input resistors RB1, RB2 and active tail current sources IC1, IC2. Differential input INA, INB is coupled to the respective input terminals of input signal source VIN, the differential input of main amplifier 110, i.e., the base of transistors QMP1, QMP2, and to compensation amplifier 120 input resistors RB1, RB2. Differential output OUTA, OUTB is coupled to the differential output of main amplifier 110, i.e., collectors of main transistors QMP1, QMP2, the differential output of compensation amplifier 120, i.e., collectors of compensation transistors QCP1, QCP2, as well as respective load resistors RL1, RL2. Active tail current sources IM1, IM2 are coupled to respective emitters of main transistors QMP1, QMP2. Active tail current sources IC1, IC2 are coupled to respective emitters of compensation transistors QCP1, QCP2. Designed to remove distortion, differential current generated by compensating amplifier 120 is subtracted from differential current generated by main amplifier 110, which is reflected in differential current through and voltage across load resistors RL1, RL2.
Main amplifier 110 and compensation amplifier 120 share the same feedback structure, although values vary. Each of main amplifier 110 and compensation amplifier 120 also share the same active tail current source structure, although values vary. Further, main amplifier 110 has load resistors RL1, RL2 while compensation amplifier 120 has input resistors RB1, RB2. While each of these features of conventional amplifier 100 were designed to solve problems, each introduces or exacerbates other problems.
For example, while input resistors RB1, RB2 are designed to increase the resistance of each base of compensation transistors QCP1, QCP2 to compensate for differences in the products of the base resistances and emitter-to-emitter capacitances of main transistors QMP1, QMP2 and compensation transistors QCP1, QCP2. Such differences adversely affect linearity at higher frequencies. However, compensation provided by input resistors RB1, RB2, is limited due to the inherent low pass RC filter characteristics created. As another example, main and compensation active tail current sources IM1, IM2, IC1, IC2, cause poor noise performance. In addition, since conventional amplifier 100 has high input impedance and since output impedance is defined by load resistors RL1, RL2, additional circuits are usually required to achieve input and output impedance matching. Still further, conventional amplifier 100 has relatively high power consumption. These problems are typical in conventional amplifiers. Thus, there is a need for a low noise, matched impedance feedback amplifier that improves linearity while reducing power consumption.